[xsl] getting a unique list using substring-after()

Hi,
I am trying to get a list of resource ids from the below XML where only one
entry with the same sub-id (numbers after the - in the @id attribute)
appears

<related>
	<content>
		<resource id="66237-89002" displayText="test1"
date_of_entry="Mar 3"/>
		<resource id="66237-89006" displayText="test1"
date_of_entry="Mar 3"/>
		<resource id="66237-87005" displayText="First entry"
date_of_entry="Mar 3"/>
		<resource id="66238-89002" displayText="test1"
date_of_entry="Mar 3"/>
		<resource id="66238-89006" displayText="test1"
date_of_entry="Mar 3"/>
		<resource id="66238-87005" displayText="First entry"
date_of_entry="Mar 3"/>
	</content>
	<product />
</related>

If I wanted to get a unique list dependent on @displayText then I could use
this code
<xsl:variable name = "unique-resources2"
select="//navigation_page/navigation/related/content/resource[not(@displayTe
xt=preceding-sibling::resource/@displayText)]"></xsl:variable>

	<resource id="66237-89002" displayText="test1" date_of_entry="Mar
3"/>
	<resource id="66237-87005" displayText="First entry"
date_of_entry="Mar 3"/>

can anyone explain why using the substring-after function
<xsl:variable name = "unique-resources"
select="//navigation_page/navigation/related/content/resource[not(substring-
after(@id,'-')=substring-after(preceding-sibling::resource/@id,'-'))]"></xsl
:variable>

yields

	<resource id="66237-89002" displayText="test1" date_of_entry="Mar
3"/>
	<resource id="66237-89006" displayText="test1" date_of_entry="Mar
3"/>
	<resource id="66237-87005" displayText="First entry"
date_of_entry="Mar 3"/>
	<resource id="66238-89006" displayText="test1" date_of_entry="Mar
3"/>
	<resource id="66238-87005" displayText="First entry"
date_of_entry="Mar 3"/>

and not
	<resource id="66237-89002" displayText="test1" date_of_entry="Mar
3"/>
	<resource id="66237-89006" displayText="test1" date_of_entry="Mar
3"/>
	<resource id="66237-87005" displayText="First entry"
date_of_entry="Mar 3"/>









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